The present invention relates to improvements in coating applicances such as for applying coating substances to both sides of a moving web, for instance a paper or a cardboard web.
In a coater, where a moving web is simultaneously coated on both sides, the supply of coating substance on the web generally takes place at the same position on both sides of the web. After this the web moves through a smoothing nip, where the excess of coating substance is removed and the coating layer is levelled. In known coaters of this kind there happens quite easily ruptures of the web especially when the web to be coated is thin and when the web is coated on one side with thick coating substance and on the other side with thin coating substance. The rupture of the web is due to two different reasons. Firstly, if the coating substance of one side is substantially thinner than the coating substance of the other side, the web is not always moving rectilinear, but there is generated waves in different points due to the different viscosity of the coating substances. This bearing causes easily ruptures of the web especially at high speeds of the web. Secondly, when supplying coating substance on the web at the same time on both sides, the liquid component of the coating substance is simultaneously absorbed in the web on its both sides, and the tensible strength of a wet web is substantially smaller than that of a dry web.
There is also known an apparatus for simultaneously doublesided coating of a web, such as disclosed in U.S. Pat. No. 4,357,370, where the coater comprises a back-up roll transmitting coating to one side of the web and a trailing blade coater applying coating on the opposite side of the web. This apparatus makes it possible to apply coatings of different kinds on each side of the web. The arrangement, however, incorporates some drawbacks when attempting to get a smooth coating of high quality on both sides of the web. Firstly, the applicators are of the short dwell type, i.e. the coater lacks a separate smoothing nip after the applying of coating on the web. Secondly, the applying of coating on the web side facing the back-up roll is carried out in a manner, that gives a satisfactory coating layer only for coatings with low quality demands, such as sizing. The centrifugal force having effect upon the coating substance fastened to the back-up roll will cause splashing of coating to the web, and the long contact surface between the web and the back-up roll has a disadvantageous effect upon the levelling of the surface layer.
The object of the inventon is to provide an improved coater for achieving a smooth and uniform coating surface of high quality on both sides of a moving web. A further object of the invention is to improve the state of function of a coater, i.e. to provide an improved coating mechanism that minimizes the risk of web rupture even at high web speeds. A still further object is to provide improvements of a coater such as the one described in the U.S. patent application Ser. No. 580,882.
The characteristic features of the invention are disclosed in claim 1. A coating appliance according to this construction achieves the object of the invention. This issues firstly from the matter, that on one side of the web the coating substance at first is spread on the surface of the back-up roll and then is transmitted from this to the web at the very smoothing nip itself, and secondly from the matter, that the both applicators are located very close to the smoothing nip. The attempt is to locate the spreading positions of the coating substance and the smoothing nip within a 30.degree. sector of the back-up roll, preferably within a 2020 sector.
In a preferred embodiment of the inventon both applicators are of the extrusion type, which through a supply slit by means of a very light excess pressure supply coating substance on the moving surfaces. It is important, that the applicator supplying coating substance on the roll is adjusted so, that the amount of coating substance supplied does not exceed the required amount of coating substance to such an extent, that an excessive pool is formed below the smoothing nip by virtue of the web moving upwards at high speed.
When the coating substance is almost pressureless in the nozzles of the applicators, the air following the coating substance will not expand since the pressure remains constant and thus uncoated areas on the web will not be caused. This phenomenon occurs in known coaters, where the moving direction of the web is opposite and the smoothing nip is formed by the bottom slit of a coating substance damming. In known coaters of this type a further drawback is the hydrostatic pressure of the damming, which rises to a remarkable degree, because the height of the damming surface has to be big enough to allow the excess of coating substance to flow away from the ends of the basin. Due to these leakages it is not possible to leave the edge zones of the web uncoated nor to arrange the control devices of the blades, nozzles etc. at the gables, which is both possible and preferable in a coater according to the invention.
Leaving the edge zones uncoated is a necessity, when for example one side is coated with surface sizing and the other side with pigment coating, when it is wanted to have different basis weights for the coatings of the different sides of the web, a task that succeeds in the best manner by changing the content of dry substance of the coating substance, or when it is wanted to leave one side wholly uncoated.
The applicator located at the web side facing the levelling blade or the like operates together with a support located at the opposite side of the web. In a favourable embodiment of the invention this support may be an adjustable rigid support such as a level blade, the surface of which is made of a plastic material having a low friction coefficient. The support may instead of a level blade comprise a support roll. The space that the support demands in the direction from the web to the back-up roll is small, and due to this the applicator in question can be placed very close to the smoothing nip.
The moving direction of the web is from below upwards, but it does not have to be exactly upright, instead it may differ somewhat from the vertical plane prior to the smoothing nip, preferably in a direction pointing away from the back-up roll. After the smoothing nip the moving direction of the web preferably differs clearly from the direction of a tangent line of the back-up roll drawn through the smoothing nip, in a direction pointing away from the back-up roll. This being the case there is generated a refracting point at the levelling blade or the like, which reduces the contact surface between the web and the back-up roll and thus improves the quality of the coating. By this arrangement also the coating on the web side facing the back-up roll will be levelled at the smoothing nip.
A further advantage in a coater according to the invention is the possibility to use the coater in the same way as the coater described in the mentioned U.S. patent application, by arranging the applicator on the roll side of the web mountable into a turned position.